Techniques for producing non-woven fabrics which use melt-adhesive composite fibers consisting of composite components having a melting point difference therebetween, and making use of their crimpability and adhesiveness, are disclosed in U.S. Pat. Nos. 3,595,731 and 3,589,956. Further, composite fibers consisting of a combination of different polyolefin components, and aiming at crimpability, but not aiming at adhesiveness due to difference of melting points, are disclosed in U.S. Pat. No. 3,505,164, No. 3,509,013, etc.
These composite fibers consisting of different component polymers cause a difference in percentage elastic shrinkage, between the two components, resulting in a large number of crimps if they are subjected to stretching operation during their production process (i.e. after stretching, crimp development occurs in a relaxed state). Further, if heat treatment is carried out making use of the difference in percentage thermal shrinkage, between two components, such composite fibers have a property of developing crimps, i.e. these composite fibers have a latent thermal crimpability. The abovementioned U.S. Pat. No. 3,505,164, U.S. Pat. No. 3,509,013, etc. propose to obtain preferable crimped composite fibers, making use of such a property. Further, U.S. Pat. No. 3,595,731 proposes obtaining a non-woven fabric having sufficient interfilamentary entanglements, by making use of their latent crimpability.
However, it has been well known that composite fibers having superior crimpability are accompanied with a great deal of shrinkage simultaneously with the development of crimps. When a web is made into a non-woven fabric, generation of crimps improves interfilamentary entanglements, yielding an elastic non-woven fabric, but, on the other hand, when a web is continuously made into a non-woven fabric, the web is accompanied with a great deal of shrinkage at the time of development of crimps, and hence the resulting web is deficient insofar as uniformity of width thickness and density are concerned. Further, when a web is subjected to heat-treatment in order to obtain a non-woven fabric like the one used for kilting, in which only the surface portion is melt-adhered interfilamentarily, there occurs a drawback in that shrinkage appears only on the surface layer, resulting in the forming of wrinkles. When such conventional composite fibers having a latent crimpability are used, it is the present status of this art that it is impossible to make the most of their characteristic feature, because of the above-mentioned drawbacks in the case of mass production, though a characteristic non-woven fabric may be obtained in case of a laboratory preparation; hence it is difficult to make them into a commercial product. According to the above-mentioned U.S. Pat. No. 3,589,956, composite fibers are annealed in a free state, before they are made into a non-woven fabric, for the purpose of preventing such a thermal shrinkage accompanying the development of latent crimps at the time of processing. However, the process of this patent is undesirable because of its complication and interfilamentary adhesion occurring at the time of annealing.
Further, in a process of producing a non-woven fabric in a wet manner, composite fibers having crimps are also undesirable because it is difficult to disperse these composite fibers uniformly in water, and hence it is impossible to obtain a uniform, wet type non-woven fabric.
The object of the present invention is to provide a process for producing polypropylene-polyethylene composite fibers which do not form crimp nor have latent crimpability almost at all and which is applicable to both wet type and dry type non-woven fabrics.